1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same and, more particularly, to a semiconductor device having an improved wiring layer mainly made of copper and a method of manufacturing the same.
2. Description of the Related Art
In recent years, as a semiconductor device is highly integrated, the width and thickness of a wiring layer are decreased, and a multi-layered structure has been developed. As a wiring material, an aluminum alloy having a low specific resistance of 2.75 .mu..OMEGA.cm and mainly made of corrosion-resistant aluminum protected from corrosion by a passivation film. However, in the wiring layer made of an aluminum alloy, since a current density is increased in accordance with a decrease in sectional area of the wiring, the wiring layer may be disconnected due to electromigration. In addition, in accordance with an increase the number of wiring layers, complicated thermal hysteresis is applied to the wiring layers, and the wiring layers are disconnected by stress migration due to thermal stress acting on the wiring layers. The above problems are caused by the following reason. Since the melting point of aluminum is low, i.e., a temperature of 660.degree. C., the diffusion of aluminum atoms, especially, atomic diffusion using a grain boundary as a path is accelerated even at a relatively low temperature.
According to the above description, a wiring layer made of copper having a melting point higher than that of aluminum by 400.degree. C. or more has been studied. Using the wiring layer made of copper (pure copper), electromigration disconnection can be suppressed compared with an aluminum-silicon-copper (Al-Si-Cu) alloy. However, it is reported that the service life of the copper wiring layer is improved by only several times or several tens of times compared with the aluminum alloy wiring layer. This service life almost corresponds to a service life estimated by activation energy of grain boundaries diffusion of copper atoms.
Generation of electromigration generally depends on the average crystal grain size and the width of a wiring layer, and the copper wiring layer is not an exception. For example, a copper film having a thickness of about 1 .mu.m formed by a conventional sputter film forming method has an average crystal grain size of about 1 .mu.m. In order to form a copper wiring layer having a so-called bamboo structure which has an electromigration resistance, the width of the wiring layer must be 0.5 .mu.m or less. When the width of the wiring layer is 0.5 .mu.m or less to improve the electromigration resistance, a practical sectional area of the wiring layer is decreased to increase its resistance. Therefore, signal transmission is delayed, and a high-speed operation is obstructed. In order to increase the sectional area of the wiring layer while keeping the width of the wiring layer to be narrow, the thickness of the wiring layer is increased. However, when the thick wiring layer is formed and covered with an insulating film, a capacitance between the adjacent wiring layers is increased to easily cause crosstalk, thereby obstructing a high-speed operation.
In addition, although an oxide production free energy of copper is slightly decreased, not only the copper is easily oxidized, but a copper oxide film formed on the copper has a low density. For this reason, when the copper wiring layer is exposed in an oxygen atmosphere, even an inner portion of the copper wiring layer is oxidized. For example, in an ashing step of removing a resist film, needle-like copper oxide is produced from the surface of the copper wiring layer, the shape of the wiring layer is degraded, and the resistance thereof is increased. In addition, in steps of depositing a passivation film and an insulating interlayer on the copper wiring layer, an adhesion strength between the wiring layer and the insulating interlayer or a passivation film is degraded due to the presence of the needle-like copper oxide to remove the films.
K. Hoshino et al. read a method of forming an improved copper wiring layer in the VMIC Conference, p226 to p232, June 12-3 1989. According to this method, an oxide film having a thickness of about 0.4 .mu.m is formed on the surface of a silicon substrate, and a tungsten layer serving as an adhesion layer and a TiN layer serving as a barrier layer are sequentially deposited on the oxide film. A Cu-Ti alloy thin film is deposited on the TiN layer by sputter evapolation using a target made of copper and titanium. After the thin film is patterned, the thin film is heat-treated in a nitrogen atmosphere. In this heat treatment (nitriding) step, Ti contained in the Cu-Ti alloy thin film pattern is diffused in the surface thereof, and the diffused Ti is nitrided to form a titanium nitride layer. As a result, the wiring layer, the peripheral surface of which is covered with the titanium nitride layer having good oxide resistance and which is made of copper including almost no Ti, is formed.
However, in the above method, as is apparent from FIG. 1 showing a relationship between a temperature of nitriding the Cu-Ti alloy thin film pattern and a specific resistance of the wiring layer, in order to obtain a wiring layer having a specific resistance of 2.5 .mu..OMEGA.cm less than the specific resistance (3 .mu..OMEGA.cm) of the Al-Si-Cu alloy, heat treatment must be performed at a temperature of 800.degree. C. As is described in the above literature, when nitriding is performed at a temperature of less than 800.degree. C., Ti is not diffused from the Cu-Ti alloy thin film, and not only a titanium nitrid layer is not formed on the surface of the wiring layer but a wiring layer having a low specific resistance almost equal to the specific resistance of copper and containing almost no Ti cannot be formed. Since the above nitriding must be performed at a relatively high temperature, an impurity diffusion layer formed on the silicon substrate in advance is subjected to diffusion again. Therefore, shallowing of diffusion layer is obstructed, and the number of electrically activated carrier is decreased.